Combined anaerobic respiration (CAD) of sewage sludge and other urban solid wastes

The UK buries about 100 million tonnes of waste a year, of which 25% is municipal solid waste (refuse). The environmental impacts from gas and leachate releases are known and direct risks to health from landfill are reported. Europe has agreed to a Landfill Directive which has set targets for the stepwise reduction in biodegradable municipal waste going to landfill. The anaerobic digestion of municipal solid waste in controlled bioreactors is an area that could play an important role in overall evolution towards sustainability by recovering biogas and organic matter. Separated hydrolysis and subsequent anaerobic codigestion was demonstrated from the literature review to have the best potential for biodegradable municipal waste diverted from landfill. The rate of hydrolysis of solids wastes remains an outstanding problem. In this research, firstly the codigestion of industrial effluent (coffee wastewater), food wastes and garden wastes were investigated for their impact on hydrolysis and digestion. The results show that there were no treatability problems for coffee wastes up to 37.5% of volume feed per day at the HRT of 9 days. The results supported the view that dilute biodegradable streams such as coffee waste may improve digestion by promoting mixing. Fruit and vegetable wastes were highly biodegradable and can have a major improvement in biogas production of the whole codigestion process, whereas garden waste was not as successful as a cosubstrate, probably because of the predominant celluloses and lignocelluloses with a low biodegradability. The literature review also revealed that washing or elutriation can remove organic matter from municipal waste. This is an important hydrolytic process in which a solubilised acidic organic matter is obtained. The codigestion of refuse hydrolysate with sewage sludge was therefore studied. A control digester treating sewage sludge only was compared with an experimental reactor fed mixed refuse hydrolysate with sewage sludge. It was possible to add the solubilised hydrolysate to existing anaerobic digesters designed at a standard sludge solids loading rate without causing overloading. (Continues...)